Step switches and tap changers are known for switching between different winding taps of a regulating transformer for voltage regulation. They have stationary contacts that are electrically connected with the individual winding taps of a regulating transformer and are usually arranged in phases on circular tracks disposed one above the other. In each horizontal plane electrical connection can be made with the stationary contacts by a movable contact, usually a contact bridge, connected with the electrical load shunt. The stationary contacts are in that case fastened to vertically extending insulating rods of hard paper or glass-fiber-reinforced synthetic material or also in the wall of insulating cylinders.
The fastening to insulating rods is hitherto usually carried out in that not only the corresponding insulating rod itself, but also a fastening part of the stationary contact are provided with respective bores and this contact arrangement is tightened together by screws and nuts. Such a contact arrangement with a screw connection is known from FIG. 1 of AT-PS 187991. However, the disadvantage of this fastening, which is simple in itself, consists in that due to the metallic fastening means penetrating the insulating rods the voltage resistance of the entire step switch or tap changer is impaired.
Numerous proposals have already become known for remedying this problem. The mentioned AT-PS 187991 itself describes a further contact arrangement in which clamping members are provided that embrace the respective insulating rods and that are screw-connected outside these insulating rods, so that no metallic fastening means penetrate the insulating rods themselves. A quite similar solution was subsequently proposed once again in AT-PS 315302. Bores through the insulating rods can indeed be avoided by that, but metallic fastening means—even if outside the insulating rods—are still present; the disadvantageous influence thereof on the voltage resistance thus continues to be present.
A further contact arrangement is known from AT-PS 382476, wherein a sleeve is provided at the contact to be fastened, the sleeve having on one side a collar with a diameter greater than the bore diameter of the respective insulating rod. On the other side of the insulating rod the sleeve of the contact to be fastened has an annular groove in which a slotted ring of plastics material is placed. However, this arrangement has not been able to gain acceptance in practice. Insulating material rods usually shrink during the necessary drying of the respective step switch or tap changer prior to being placed in operation; in the case of this known contact arrangement the individual contacts then loosen due to the fastening purely by shape-locking coupling, which is not desirable.
A contact arrangement as well as an associated fastening method are known from DE-PS 3801151 and 3801152, wherein the contact is held by permanent deformation of its sleeve on the outer side, stated more precisely by expansion of bores specifically provided for that purpose. This is relatively costly and requires a special tool for expanding the described bores and beyond that similarly enables only fastening in a shape-locking manner. Finally, disassembly of this contact arrangement without destruction is also not possible.
DE-GM 9010730 proposes a similar contact arrangement; in this case the sleeve of the contact is weakened in its cross-section in a defined region and a bulge-like thickening of the sleeve is achieved at this location by upsetting from the outside, whereby fixing is effected. A special tool is indeed not required for the described upsetting from outside—a hammer suffices—but here, too, there is the disadvantage that, apart from the connection which is again only a shape-locking coupling, disassembly is similarly possible only by destruction of the contact.
DE-PS 4236528 (U.S. Pat. No. 5,374,781) describes a further possibility of fastening a contact by a transverse pin that penetrates the contact sleeve, and locking means co-operating therewith. This connection is detachable without destruction, can also be executed to be force-locking and is also suitable for simultaneous fastening of shield rings. Nevertheless, it was not able gain acceptance, because it is of relatively complicated construction and requires a multiplicity of individual parts produced with a high degree of accuracy relative to one another.